Systems and methods for production of silicon using a horizontal magnetic field

ABSTRACT

A method for producing a silicon ingot by the horizontal magnetic field Czochralski method includes rotating a crucible containing a silicon melt, applying a horizontal magnetic field to the crucible, contacting the silicon melt with a seed crystal, and withdrawing the seed crystal from the silicon melt while rotating the crucible to form a silicon ingot. The crucible has a wettable surface with a cristobalite layer formed thereon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claim priority to U.S. Provisional Patent ApplicationNo. 62/947,785 filed Dec. 13, 2019, the entire disclosure of which ishereby incorporated by reference in its entirety.

FIELD

This disclosure generally relates to the production of silicon ingots,and more specifically, to methods and systems for achieving a highsuccess ratio producing silicon ingots in the Czochralski process usinga horizontal magnetic field.

BACKGROUND

During the 1990's, at least some high quality silicon growth was mainlycontrolled by the thermal condition of the puller and more specificallythe hot zone (HZ) design itself, because the ratio of the pulling speedto the thermal gradient (v/G) was considered the dominant factor. In thelate 1990's, further consideration of the crystal/melt interface at thesame v/G was included in the growth of at least some high qualitysilicon. At that time, application of high quality silicon to memorydevices really expanded as more customers transitioned from epi topolished and from 200 mm to 300 mm silicon. Soon after, it becameestablished that high quality silicon growth requires very stableprocess growth conditions and controlled melt flow to achieve thespecific crystal/melt needed to achieve the desired low crystaldefectivity during growth.

Peripherally, as silicon crystal growth transitioned from 200 mm to 300mm and corresponding charge sizes increased to maintain productivity,the need for magnetic field application to stabilize melt flow in theincreasing melt volume was recognized as a dominate feature.

Several silicon manufactures transitioned to a horizontal magnetic fieldCzochralski process (HMCZ) in the early 2000's when high quality 300 mmsilicon production started in order to control the crystal/meltinterface effectively. Other silicon manufactures used a cusp magneticfield for 300 mm production of high quality silicon. In both cases,magnetic field in the silicon melt had a dramatic impact on crystalquality and performance and every manufacturer developed their owntechnique to optimize performance and quality from the onset.

During the process of producing single crystal silicon ingots with theCZ process and a magnetic field, oxygen may be introduced into siliconcrystal ingots through a melt-solid or melt crystal interface. Theoxygen may cause various defects in wafers produced from the ingots,reducing the yield of semiconductor devices fabricated using the ingots.For example, memory devices, insulated-gate bipolar transistors (IGBTs),high quality radio-frequency (RF), high resistivity silicon on insulator(HR-SOI), and charge trap layer SOI (CTL-SOI) applications typicallyrequire a low interstitial oxygen concentration (Oi) in order to achievehigh resistivity. In the case of HMCZ process, it was believed that theprocess typically requires a very low crucible rotation (C/R) to controloxygen in the growing crystal, particularly to control oxygen inclusionto the desired range applicable for memory devices. Further, a higheroccurrence ratio of lost zero dislocation (LZD) from the crown to theend of body was found in HMCZ as compared to processes using a cuspmagnetic field.

Thus, there exists a need for methods and systems that reduce LZD losseswith HMCZ growth and provide an improved ZD success ratio for highquality silicon growth from crown to body.

This background section is intended to introduce the reader to variousaspects of art that may be related to various aspects of the presentdisclosure, which are described and/or claimed below. This discussion isbelieved to be helpful in providing the reader with backgroundinformation to facilitate a better understanding of the various aspectsof the present disclosure. Accordingly, it should be understood thatthese statements are to be read in this light, and not as admissions ofprior art.

BRIEF SUMMARY

In one aspect of this disclosure, a method for producing a silicon ingotby the horizontal magnetic field Czochralski method includes rotating acrucible containing a silicon melt, applying a horizontal magnetic fieldto the crucible, contacting the silicon melt with a seed crystal, andwithdrawing the seed crystal from the silicon melt while rotating thecrucible to form a silicon ingot. The crucible has a wettable surfacewith a cristobalite layer formed thereon

Another aspect is a wafer generated from a silicon ingot produced usingthe method described above.

Another aspect is a system for producing a silicon ingot. The systemincludes a crucible to contain a silicon melt, magnetic poles to producea horizontal magnetic field, and a controller. The crucible has awettable surface with a cristobalite layer formed thereon. Thecontroller is programmed to produce a silicon ingot by rotating thecrucible containing the silicon melt, applying a horizontal magneticfield to the crucible using the magnetic poles, contacting the siliconmelt with a seed crystal, and withdrawing the seed crystal from thesilicon melt while rotating the crucible to form a silicon ingot.

Various refinements exist of the features noted in relation to theabove-mentioned aspect. Further features may also be incorporated in theabove-mentioned aspect as well. These refinements and additionalfeatures may exist individually or in any combination. For instance,various features discussed below in relation to any of the illustratedembodiments may be incorporated into the above-described aspect, aloneor in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a top view of a crucible of one embodiment.

FIG. 2 is a side view of the crucible shown in FIG. 1 .

FIG. 3 is a schematic illustrating a horizontal magnetic field appliedto a crucible containing a melt in a crystal growing apparatus.

FIG. 4 is a block diagram of a crystal growing system.

FIG. 5 presents temperature field in melt free surface by MGP in mm andcrucible rotation in RPM.

FIG. 6 is a graph of crystallization rate as a function of time.

FIG. 7 is a graph of the thickness of the layer formed as a function oftime at a temperature of 1360° C.

FIG. 8 is a contour plot of ZD success ratio by crucible rotation andthe quantity of melt modifier in a natural sand crucible.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring initially to FIGS. 1 and 2 , a crucible of one embodiment isindicated generally at 10. A cylindrical coordinate system for crucible10 includes a radial direction R 12, an angular direction θ 14, and anaxial direction Z 16. The crucible 10 contains a melt 25 having a meltsurface 36. A crystal 27 is grown from the melt 25. The melt 25 maycontain one or more convective flow cells 17, 18 induced by heating ofthe crucible 10 and rotation of the crucible 10 and/or crystal 27 in theangular direction θ 14. The structure and interaction of these one ormore convective flow cells 17, 18 are modulated via regulation of one ofmore process parameters and/or the application of a magnetic field asdescribed in detail herein below.

FIG. 3 is a diagram illustrating a horizontal magnetic field beingapplied to crucible 10 containing melt 25 in a crystal growingapparatus. As shown, crucible 10 contains silicon melt 25 from which acrystal 27 is grown. The transition between the melt and the crystal isgenerally referred to as the crystal-melt interface (alternatively themelt-crystal, solid-melt or melt-solid interface) and is typicallynon-linear, for example concave, convex or gull-winged relative to themelt surface. Two magnetic poles 29 are placed in opposition to generatea magnetic field generally perpendicular to the crystal-growth directionand generally parallel to the melt surface 36. The magnetic poles 29 maybe a conventional electromagnet, a superconductor electromagnet, or anyother suitable magnet for producing a horizontal magnetic field of thedesired strength. Application of a horizontal magnetic field gives riseto Lorentz force along axial direction, in a direction opposite of fluidmotion, opposing forces driving melt convection. The convection in themelt is thus suppressed, and the axial temperature gradient in thecrystal near the interface increases. The melt-crystal interface thenmoves upward to the crystal side to accommodate the increased axialtemperature gradient in the crystal near the interface and thecontribution from the melt convection in the crucible decreases. Thehorizontal configuration has the advantage of efficiency in damping aconvective flow at the melt surface 36.

FIG. 4 is a block diagram of a crystal growing system 100. System 100employs a Czochralski crystal growth method to produce a semiconductoringot. In this embodiment, system 100 is configured to produce acylindrical semiconductor ingot having an ingot diameter of one-hundredand fifty millimeters (150 mm), greater than one-hundred fiftymillimeters (150 mm), more specifically in a range from approximately150 mm to 460 mm, and even more specifically, a diameter ofapproximately three-hundred millimeters (300 mm). In other embodiments,system 100 is configured to produce a semiconductor ingot having atwo-hundred millimeter (200 mm) ingot diameter or a four-hundred andfifty millimeter (450 mm) ingot diameter. In addition, in oneembodiment, system 100 is configured to produce a semiconductor ingotwith a total ingot length of at least nine hundred millimeters (900 mm).In some embodiments, the system is configured to produce a semiconductoringot with a length of one thousand nine hundred and fifty millimeters(1950 mm), two thousand two hundred and fifty millimeters (2250 mm), twothousand three hundred and fifty millimeters (2350 mm), or longer than2350 mm. In other embodiments, system 100 is configured to produce asemiconductor ingot with a total ingot length ranging from approximatelynine hundred millimeters (900 mm) to twelve hundred millimeters (1200mm), between approximately 900 mm and approximately two thousandmillimeters (2000 mm), or between approximately 900 mm and approximatelytwo thousand five hundred millimeters (2500 mm). In some embodiments,the system is configured to produce a semiconductor ingot with a totalingot length greater than 2000 mm.

The crystal growing system 100 includes a vacuum chamber 101 enclosingcrucible 10. A side heater 105, for example, a resistance heater,surrounds crucible 10. A bottom heater 106, for example, a resistanceheater, is positioned below crucible 10. During heating and crystalpulling, a crucible drive unit 107 (e.g., a motor) rotates crucible 10,for example, in the clockwise direction as indicated by the arrow 108.Crucible drive unit 107 may also raise and/or lower crucible 10 asdesired during the growth process. Within crucible 10 is silicon melt 25having a melt level or melt surface 36. In operation, system 100 pulls asingle crystal 27, starting with a seed crystal 115 attached to a pullshaft or cable 117, from melt 25. One end of pull shaft or cable 117 isconnected by way of a pulley (not shown) to a drum (not shown), or anyother suitable type of lifting mechanism, for example, a shaft, and theother end is connected to a chuck (not shown) that holds seed crystal115 and crystal 27 grown from seed crystal 115.

Crucible 10 and single crystal 27 have a common axis of symmetry 38.Crucible drive unit 107 can raise crucible 10 along axis 38 as the melt25 is depleted to maintain melt level 36 at a desired height. A crystaldrive unit 121 similarly rotates pull shaft or cable 117 in a direction110 opposite the direction in which crucible drive unit 107 rotatescrucible 10 (e.g., counter-rotation). In embodiments using iso-rotation,crystal drive unit 121 may rotate pull shaft or cable 117 in the samedirection in which crucible drive unit 107 rotates crucible 10 (e.g., inthe clockwise direction). Iso-rotation may also be referred to as aco-rotation. In addition, crystal drive unit 121 raises and lowerscrystal 27 relative to melt level 36 as desired during the growthprocess.

According to the Czochralski single crystal growth process, a quantityof polycrystalline silicon, or polysilicon, is charged to crucible 10. Aheater power supply 123 energizes resistance heaters 105 and 106, andinsulation 125 lines the inner wall of vacuum chamber 101. A gas supply127 (e.g., a bottle) feeds argon gas to vacuum chamber 101 via a gasflow controller 129 as a vacuum pump 131 removes gas from vacuum chamber101. An outer chamber 133, which is fed with cooling water from areservoir 135, surrounds vacuum chamber 101.

The cooling water is then drained to a cooling water return manifold137. Typically, a temperature sensor such as a photocell 139 (orpyrometer) measures the temperature of melt 25 at its surface, and adiameter transducer 141 measures a diameter of single crystal 27. Inthis embodiment, system 100 does not include an upper heater. Thepresence of an upper heater, or lack of an upper heater, alters coolingcharacteristics of crystal 27.

Magnetic poles 29 are positioned outside the vacuum chamber 101 toproduce a horizontal magnetic field (shown in FIG. 3 ). Althoughillustrated approximately centered on the melt surface 36, the positionof the magnetic poles 29 relative to the melt surface 36 may be variedto adjust the position of the maximum gauss plane (MGP) relative to themelt surface 36. A reservoir 153 provides cooling water to the magneticpoles 29 before draining via cooling water return manifold 137. Aferrous shield 155 surrounds magnetic poles 29 to reduce stray magneticfields and to enhance the strength of the field produced.

A control unit 143 is used to regulate the plurality of processparameters including, but not limited to, at least one of crystalrotation rate, crucible rotation rate, and magnetic field strength. Invarious embodiments, the control unit 143 may include a memory 173 andprocessor 144 that processes the signals received from various sensorsof the system 100 including, but not limited to, photocell 139 anddiameter transducer 141, as well as to control one or more devices ofsystem 100 including, but not limited to: crucible drive unit 107,crystal drive unit 121, heater power supply 123, vacuum pump 131, gasflow controller 129 (e.g., an argon flow controller), magnetic polespower supply 149, and any combination thereof. The memory 173 may storeinstructions that, when executed by the processor 144 cause theprocessor to perform one or more of the methods described herein. Thatis, the instructions configure the control unit 143 to perform one ormore methods, processes, procedures, and the like described herein.

Control unit 143 may be a computer system. Computer systems, asdescribed herein, refer to any known computing device and computersystem. As described herein, all such computer systems include aprocessor and a memory. However, any processor in a computer systemreferred to herein may also refer to one or more processors wherein theprocessor may be in one computing device or a plurality of computingdevices acting in parallel. Additionally, any memory in a computerdevice referred to herein may also refer to one or more memories whereinthe memories may be in one computing device or a plurality of computingdevices acting in parallel. Further, the computer system may locatednear the system 100 (e.g., in the same room, or in an adjacent room), ormay be remotely located and coupled to the rest of the system via anetwork, such as an Ethernet, the Internet, or the like.

The term processor, as used herein, refers to central processing units,microprocessors, microcontrollers, reduced instruction set circuits(RISC), application specific integrated circuits (ASIC), logic circuits,and any other circuit or processor capable of executing the functionsdescribed herein. The above are examples only, and are thus not intendedto limit in any way the definition and/or meaning of the term“processor.” The memory may include, but is not limited to, randomaccess memory (RAM) such as dynamic RAM (DRAM) or static RAM (SRAM),read-only memory (ROM), erasable programmable read-only memory (EPROM),electrically erasable programmable read-only memory (EEPROM), andnon-volatile RAM (NVRAM).

In one embodiment, a computer program is provided to enable control unit143, and this program is embodied on a computer readable medium. Thecomputer readable medium may include the memory 173 of the control unit143. In an example embodiment, the computer system is executed on asingle computer system. Alternatively, the computer system may comprisemultiple computer systems, connection to a server computer, a cloudcomputing environment, or the like. In some embodiments, the computersystem includes multiple components distributed among a plurality ofcomputing devices. One or more components may be in the form ofcomputer-executable instructions embodied in a computer-readable medium.

The computer systems and processes are not limited to the specificembodiments described herein. In addition, components of each computersystem and each process can be practiced independent and separate fromother components and processes described herein. Each component andprocess also can be used in combination with other assembly packages andprocesses.

In one embodiment, the computer system may be configured to receivemeasurements from one or more sensors including, but not limited to:temperature sensor 139, diameter transducer 141, and any combinationthereof, as well as to control one or more devices of system 100including, but not limited to: crucible drive unit 107, crystal driveunit 121, heater power supply 123, vacuum pump 131, gas flow controller129 (e.g., an argon flow controller), magnetic poles power supply 149,and any combination thereof as described herein and illustrated in FIG.4 in one embodiment. The computer system performs all of the steps usedto control one or more devices of system 100 as described herein.

The loss of zero dislocation (ZD) structure (quantified by LZD rate) isgenerally higher during silicon crystal Cz growth in a horizontal magnet(HMCZ) field versus growth in a Cusp (or vertical) magnetic field.However, the LZD rate in HMCZ can be lowered dramatically if the crystalis grown in a synthetic lined crucible versus a natural sand linedcrucible. But while the ZD rate is better, the cost of synthetic linedcrucibles is higher than natural sand. Additionally, the thin (roughly 2mm thickness) synthetic liner dissolves in a relatively short processtime, leaving the backing sand layer exposed to the melt, which allowsquartz particles to enter the melt and hit the growing crystal. Thus,the exposure of bubbles in the liner or even from the backing sand intothe melt is higher. To avoid bubble exposure and/or backing sandcontamination by dissolution when using synthetic lined crucibles, theprocess hot hours are generally limited to less than ˜250 hours, whichis much shorter process time than achievable for natural sand crucibles(approximately 400-500 hours or more). Because the hot time for crystalgrowth depends on the process conditions, HZ configuration, andattempts, recharge capability and multiple rod growth per batch can beimpacted using synthetic lined crucibles. Therefore, silicon growthusing HMCZ in synthetic lined crucibles generally requires optimizationof the crucible condition to ensure the best ZD rate and lowest attemptsso that maximum recharge capability can be achieved.

Further, a horizontal magnet in Cz growth enhances the melt flow with anirregular velocity melt wave continuously knocking against the cruciblewall surface with a strong force in a transient behavior. In this case,the crucible surface condition is very critical for quartz piecegeneration which directly relates to the ZD success. This is illustratedin FIG. 5 , in which the melt flow and temperature field of the melt aresignificantly impacted by the magnetic field strength and MGP position.As shown in the figure, the temperature variation of the circumferentialdirection at the melt surface is markedly changed by the magnetic fielddirection.

These and other difficulties may be overcome or mitigated in embodimentsof the present disclosure through use of one or both of two techniquesdescribed in detail below. Generally, in the first aspect, acristobalite layer is formed on the wettable surface of the interior ofthe crucible. The wettable surface generally refers to the surface ofthe crucible that may be in contact with the melt during siliconproduction. The wettable surface generally includes the interior bottomof the crucible, at least a portion of the interior sidewalls of thecrucible, and the interior portions connecting the interior sidewallsand bottom of the crucible. In FIG. 2 , the wettable surface is all ofthe interior surface of the crucible 10 below and including the meltsurface 36. The wettable surface may also extend above the melt surface36. The second technique described in this disclosure is to increase therotational speed of the crucible.

In general, a strong and unsteady melt flow is induced by an HMCZmagnetic field and this can generate strong thermo-mechanical stress andmechanical impact on the crucible wall causing quartz particlegeneration. However, high crucible rotation (C/R) will produce fasterconvective flow near the crucible wall surface, which can interfere withthe melt flow driven by a magnetic field. This will reduce the stressand impact on the wall surface. Consequently, the generation of quartzparticles at the wall surface is reduced which in turn reduces LZDsduring crystal growth.

LZDs may also be reduced by enabling the formation of a generallyuniform crystalline SiO2 layer (referred to as cristobalite layer) onthe crucible's wettable surface. This layer is more stable and strongerthan amorphous quartz itself, and thereby it is more resistant to meltattack by stress or mechanical impact. Thus, the cristobalite layerreduces quartz particle generation.

There are at least two methods to promote crystalline layer growth onquartz crucibles. The first method is to use pre-coated cruciblesprecoated with a compound, such as BaOH, that will promote cristobalitegrowth, and the other is to add a suitable melt modifier (MM) into themelt before crystal growth. Nonlimiting examples of suitable MMs includebarium (Ba) and strontium (Sr). More specifically, nonlimiting exampleof suitable MMs include barium carbonate (BaCO3), barium oxide (BaO),and strontium carbonate (SrCO3).

Cristobalite formation on the amorphous quartz inner wall is governed bypressure, Oi concentration, H2O and hydrogen content, temperature, andthe like. As shown in FIGS. 6 and 7 , the formation and growth of acrystalline layer on the crucible wall is governed by the temperature ofthe crucible wall and concentration of the MM, which is consumed by thecrucible. FIG. 6 compares the crystallization rate as a function of timefor no MM and an 8% AL2O3 MM. FIG. 7 compares the thickness of the layerformed as a function of time at a temperature of 1360° C. for no MM, an8% AL2O3 MM, and a barium based MM. These charts indicates that a properMM addition (e.g., a barium based MM) into the melt or a pre-coating ofa barium compound generates a uniform and thick crystalline (i.e.,cristobalite) layer on the wall of natural sand crucibles giving similarbehavior and performance to synthetic lined crucibles. Because thecristobalite layer has slower dissolution rate than the fused quartz,the generation of quartz pieces by thermo or mechanical stress isdecreased so the crystal LZD occurrence is decreased thereafter.Further, the generation of secondary bubbles in the Bubble Free Layer(BFL) of the natural sand crucible and its propagation into the melt ismuch less than that of synthetic lined crucible in general due to thedifference of material properties.

The formation and growth of a uniform cristobalite layer either throughpre-coating on the crucible or post addition of melt modifier into themelt is started during stabilization mode prior to crystal growth (i.e.after melting of poly silicon or during the melting). In case of a postMM addition, the MM is introduced after meltdown, so the formation speedof the cristobalite is slower than otherwise pre-coated case. However,post addition of the MM can yield lower air pocket (APK) losses becausebubbles formed and trapped at the crucible wall can be released to thesurface prior to the formation of the stable cristobalite layer. Becauseit typically takes 3˜7 hours from the start of the stabilization step tothe start of body growth, the cristobalite thickness is estimated to begreater than about 2 mm with an appropriate amount of MM added toincrease the formation rate of the cristobalite. In practice, acristobalite layer of less than about 1.0 mm is typically formed on thesurface of the crucible (whether natural sand or synthetic). Somespecial case, such as heavily doped process for P++ or N++, yield athicker cristobalite layer by adding a large amount of MM. In such casesa cristobalite layer of approximately 1.0 mm (+/−) is formed. Thesethicknesses differ from the 2.0 mm because although the cristobalitelayer is grown by hot time as shown in FIG. 7 , wet cristobalite layeris continuously being dissolved to melt. Other embodiments include acristobalite layer of approximately 2.0 mm, greater than 1.5 mm, greaterthan 1.0 mm, greater than 0.75 mm, greater than 0.5 mm, or greater than0.25 mm formed on the wettable surface of the crucible. In someembodiments the cristobalite layer is less than 3.0 mm thick, less than2.0 mm thick, less than 1.25 mm thick, or less than 1.0 mm thick. Insome embodiments, the cristobalite layer falls within a range defined bythe minima and maxima above, such as between 0.25 mm and 1.25 mm.Generally, too thin a cristobalite layer may be insufficient to providethe benefits described herein, while too thick a cristobalite layer maybe more likely to break and enter the melt (potentially contributing toan LZD) during silicon production.

Melt modifier addition described above forms a uniform crystalline layerat the crucible wettable surface and this crystalline structure has astrong resistance against the thermomechanical stress induced by theirregular (transient) melt flow produced during HMCZ. The formation of athick and uniform cristobalite can resist the stress and impact from themelt flow, reducing crucible surface damage (i.e., resisting damagegenerating quartz particles in the melt) which will increase the ZDsuccess.

As mentioned previously, a strong convective flow caused by highercrucible rotation will reduce the variation of temperature in the meltfree surface and reduce the melt flow induced by a horizontal magnet asseen in Case 6 in FIG. 5 . In case 6, C/R is 1.6 RPM as compared to 0.6RPM in cases 1-5 and 7. In other words, the force produced by themagnetic field from the melt to the crucible wall surface is reduced orblocked by the convective flow related with higher crucible rotation.Thereby, the possibility of quartz piece generation due to damage ofcrucible wall surface is decreased, also improving the potential of ZDsuccess.

A test condition was performed to understand ZD success and on bothsynthetic lined and natural sand crucibles in a horizontal magnet as afunction of crucible rotation speed and the quantity of melt modifieraddition. The synthetic crucible showed high ZD success ratio across abroad range of crucible rotation and MM. For the natural sand linedcrucible, a total of 96 trials with 19 different conditions werecompleted and the results are summarized with a contour plot in FIG. 8 .The data for FIG. 8 was from approximately 200 mm body length toapproximately OE of the crystal. In FIG. 8 , CR is the crucible rotationin RPM, a Success %_3 of 0.0 is a ZD fail (LZD), and a Success % 3 of1.0 is 100% ZD success. The sign of the crucible rotation indicatesdirection of rotation of the crucible. For some pulls, the cruciblerotation ramped between speeds during portions of the pull, an arbitraryvalue of crucible rotation was selected for plotting, and the data wascollected from greater than 1000 mm body length to approximately OE ofthe crystal. Results clearly show that higher ZD success ratio isachieved with increasing the absolute value of the speed of cruciblerotation and quantity of melt modifier (in this case, Ba based).

The use of increased speed of crucible rotation and the creation of acristobalite layer are shown to individually and in combination resultin improved ZD outcomes. At lower C/R, such as between about 0 and about2 RPM, addition of suitable MM (or use of precoated crucible) is neededto increase the ZD success ratio. As the C/R is increased to betweenabout 2 RPM and about 5 RPM, the amount of MM needed to gain anequivalent success rate decreases and may even be zero MM. Within thisrange, additional gains in ZD success ratio may be obtained through useof at least some MM. When the C/R exceeds about 5 RPM there is generallyno ZD concern from the crucible and no MM is likely to be needed.However, at such speeds, additional process condition will likely berequired to control the quality of the products, because it may haveother issues such as oxygen control due to melt flow velocity, meltlevel control caused by centrifugal force, and the like.

Thus, some embodiments of the present disclosure use a MM more than 1.7grams per square meter of the wettable surface area of crucible duringan HMCZ process at any C/R rate to improve the ZD success rate. In someembodiments, a MM between 1.7 and 2.0 grams/m² of the wettable surfaceof the crucible is used. In still other embodiments, a MM between 1.7and 5.4 grams/m² of the wettable surface of the crucible is used. Instill other embodiments, a MM greater than 5.4 grams/m² of the wettablesurface of the crucible may be used, but the large amount of MM mightcause LZD in multiple recharge processes. The quantities above (e.g.,1.7 grams/m²) are based on BaCO3 as the MM. Similar embodiments usingBaO or SrCO3 include an amount of the particular MM functionallyequivalent to the amount of BaCO3.

Some embodiments use a MM between about 0 and 0.5 g/m² and a cruciblerotation greater than about 2.0 RPM. In some such embodiments, thecrucible is a natural sand crucible. Alternatively, the crucible may bea synthetic crucible.

Embodiments of the methods described herein achieve superior resultscompared to prior methods and systems. For example, the methodsdescribed herein facilitate producing silicon with a higher ZD successrate than some other methods.

When introducing elements of the present invention or the embodiment(s)thereof, the articles “a”, “an”, “the” and “said” are intended to meanthat there are one or more of the elements. The terms “comprising”,“including” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately,” and “substantially,” is notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged; such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.

As various changes could be made in the above without departing from thescope of the invention, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A method for producing a silicon ingot, themethod comprising: rotating a natural sand crucible containing a siliconmelt, the crucible having an inner wall with a cristobalite layer formedthereon at more than two revolutions per minute, the cristobalite layerhaving a thickness of more than between 0.25 mm and 1.25 mm; applying ahorizontal magnetic field to the crucible; contacting the silicon meltwith a seed crystal; and withdrawing the seed crystal from the siliconmelt while rotating the crucible between two revolutions per minute andfive revolutions per minute to form the silicon ingot.
 2. The method ofclaim 1, wherein the method further comprises: adding solid-phasepolycrystalline silicon to the natural sand crucible; and heating thepolycrystalline silicon to form the silicon melt.
 3. The method of claim2, further comprising adding a melt modifier to the natural sandcrucible to form the cristobalite layer on the inner wall of the naturalsand crucible.
 4. The method of claim 3, wherein adding the meltmodifier comprises adding the melt modifier while heating thepolycrystalline silicon to form the silicon melt.
 5. The method of claim3, wherein adding the melt modifier comprises adding the melt modifierafter the silicon melt has been formed.
 6. The method of claim 3,wherein adding the melt modifier comprises adding barium carbonate(BaCO₃).
 7. The method of claim 6, wherein adding barium carbonatecomprises adding more than 1.7 grams per square meter of the inner wallof the natural sand crucible.
 8. The method of claim 3, wherein addingthe melt modifier comprises adding one of barium oxide (BaO) orstrontium carbonate (SrCO₃).
 9. The method of claim 2, wherein addingsolid-phase polycrystalline silicon to the natural sand cruciblecomprises adding solid-phase polycrystalline silicon to the natural sandcrucible having a barium based coating already formed on the inner wallof the natural sand crucible.
 10. A wafer from a silicon ingot producedaccording to the method of claim 1.